844 NATURAL HISTORY OF AQUATIC ANIMALS. 



skeleton made up of calcareous spicules, while the fossil forms referred to the same have calcareous 

 columnar supports, instead of spicular. 



The soft and fleshy part of the Sponge, which is the truly organized portion, and upon a 

 knowledge of which we must rely for a perfect understanding of the relations of .Sponges, is the 

 most difficult to study, as it is also the least known. It collapses and begins 1o decay almost on 

 the moment of the Sponge being taken from the water, and alcoholic preparations are of compara- 

 tively little value for investigation. The structure of some forms has, however, been sufficiently 

 well made out to give us a tolerably clear idea of what it must be in the entire group. Prof. A. 

 Hyatt describes the general structure of the Sponge as follows: 1 



4i They are structurally remarkably uniform, though differing greatly in external aspect. They 

 consist internally of a mass or layer of sarcode or mesoderm, containing a greater or less number 

 of true cells, and have an ectoderm and endoderin of cellular tissue. The majority of the forms 

 are supported by a skeleton of interwoven threads or spicules, or both, of various forms. The 

 exterior is perforated by innumerable pores, leading into channels in the interior, which enlarge 

 and join with groups of neighboring channels, forming large branches. These, in turn, form 

 junctions with other branches, and finally all of them unite, into one or several large trunks, 

 which open outwards, like minute craters, on the external surface. These are lined with another 

 membrane, differing from anything else of its kind in the animal kingdom. It is composed of 

 minute cells, furnished on the free side with a long whip or flagellum, surrounded by a collar. 

 Their interiors contain .1 nucleus and digestive vacnoles, and they, in all respects, resemble the- 

 independent animals known as flagellate infusoria. They take in and digest food in the same 

 manner, and eject excrements in great profusion from the area inclosed by the membraneous collar. 



"The eggs and spermatozoa are derived from moditied cells of the mesoderm, whereas the 

 skeleton is either built up partly from the external membrane, and partly from the sarcode by 

 exogenous growth, or by the transformation of the loose cells of the sarcode into spicuhi'. The 

 function of the smaller external pores is to admit the water, which is thus strained and deprived 

 ol' its coarser floating material. It. is then carried along the canals, by the motion of the cilia, 

 and conveys its load of minute food to the ampnllaceous sacs and zooidal cells. The hydraulic 

 pressure occasioned by the inward flow of the innumerable minute streams forces it through the 

 larger trunks and out at the craters or ostioles with great rapidity." 



Their peculiar cellular structure caused the Sponges when they were first carefully studied to 

 be looked upon as compound animals, but this idea has been refuted by more recent studies, and 

 each individual Sponge is now considered, "in its simplest adult form, as homologically a single 

 animal with the internal structure and functions of a colonial organization." 



The branch or subkingdom Par if era is divided into two classes, the Calci Spongiee (calcareous 

 Sponges) and the Carneo-Sponfii(e (horny and siliceous Sponges). 



The Otbt-SffHfia are again divided into two orders, and the Carneo-Spongia; into four orders, 

 the Halixarcoidea, Keratoidea, Kerato-Siiicioidea, and Silitioi<lc. 



The Keratoidea includes all the purely horny Sponges, and the only genus, Spongia, of direct 

 importance to mankind. According to Professor Hyatt, the horny Sponges "appear to require 

 for the production of the forms in abundance tropical or subtropical seas, and obtain by far their 

 greatest, development in the number of the forms and species in the West Indian seas. The 

 typical forms, the commercial Sponges, are essentially confined to the waters of the Caribbean 

 Islands, Bahama Archipelago, and the southern and western coasts of Florida, in this hemisphere. 

 and to the Mediterranean and Hed Seas in the other. 



'Memoirs Boston Soc. Nat. Hist., ii, 187r>-'77. 



